In the field of semiconductor processing, photolithography is a widely employed technique to “pattern”, i.e. define a profile in one or more layer of semiconductor material, a semiconductor wafer. Using this technique, hundreds of Integrated Circuits (ICs) formed from an even larger number of transistors can be formed on a wafer of silicon. In this respect, for each wafer, the ICs are formed one at a time and on a layer-by-layer basis.
For about the last four decades, a photolithography apparatus, sometimes known as a cluster or photolithography tool, has been employed to carry out a photolithographic process. The cluster comprises a track unit that prepares the wafer, including providing layers of photosensitive material on the surface of the wafer prior to exposure to a patterned light source. To expose the wafer to the patterned light source, the wafer is transferred to an optics unit that is also part of the cluster. The patterned light source is generated by passing a beam of light through a chrome-covered mask, the chrome having been patterned with an image of a given layer of an IC to be formed, for example, transistor contacts. Thereafter, the wafer is returned to the track unit for subsequent processing including development of the layers of photosensitive material mentioned above.
The wafer, carrying the layers of photosensitive material, is supported by a movable stage. A projection lens focuses the light passing through the mask to form an image on a first field over the layers of photosensitive material where an IC is to be formed, exposing the field of the layers of photosensitive material to the image and hence “recording” the pattern projected through the mask. The image is then projected on another field over the layers of photosensitive material where another IC is to be formed, this field over the layers of photosensitive material being exposed to the projected image, and hence pattern.
The above process is repeated for other fields where other ICs are to be formed. Thereafter, the wafer is, as mentioned above, returned to the track unit, and the exposed layers of photosensitive material, which become soluble or insoluble through exposure depending upon the photosensitive materials used, are developed to leave a “photoresist” pattern corresponding to a negative (or positive) of the image of a layer of one or more ICs to be created. After development, the wafer undergoes various other processes, for example ion implantation, etching or deposition. The remaining layers of photosensitive material are then removed and fresh layers of photosensitive material are subsequently provided on the surface of the wafer depending upon particular application requirements for patterning another layer of the one or more ICs to be formed.
In relation to the patterning process, the resolution of a photolithography apparatus, or scanner, impacts upon the width of wires and spaces therebetween that can be “printed”, the resolution being dependent upon the wavelength of the light used and inversely proportional to a so-called “numerical aperture” of the scanner. Consequently, to be able to define very high levels of detail a short wavelength of light is required and/or a large numerical aperture.
The numerical aperture of the scanner is dependent upon the product of two parameters. A first parameter is the widest angle through which light passing through the lens can be focused on the wafer, and a second parameter is the refractive index of the medium through which the light passes when exposing the layers of photosensitive material on the wafer.
Indeed, to provide the increased resolution demanded by the semiconductor industry, it is known to reduce wavelengths of light used whilst also making lenses bigger to increase the numerical aperture. However, practical limits to the usable wavelengths of light are currently being reached, for example due to cost of having to use lenses formed from different materials compatible with the lower wavelengths of light, and scarcity of suitable lens materials.
Additionally, the above-described scanner operates in air, air having a refractive index of 1, resulting in the scanner having a numerical aperture between 0 and 1. Since the numerical aperture needs to be as large as possible, and the amount the wavelength of light can be reduced is limited, an improvement to the resolution of the scanner has been proposed that, other than by increasing the size of the lens, uses the scanner in conjunction with a medium having a refractive index greater than that of air, i.e. greater than 1. In this respect, the more recent photolithographic technique proposed, employing water and known as immersion lithography, can achieve higher levels of device integration than can be achieved by air-based photolithography techniques.
Therefore, scanners employing this improvement (immersion scanners) continue to use low wavelengths of light, but the water provides a refractive index of approximately 1.4 at a wavelength of 193 nm between the lens and the wafer, thereby achieving increased depth of focus and effectively increasing the numerical apertures of the immersion lithographic apparatus.
Further, the refractive index of the water is very close to that of quartz from which some lenses are formed, resulting in reduced refraction at the interface between the lens and the water. The reduced refraction allows the size of the lens to be increased, thereby allowing advantage to be taken of the higher available numerical aperture.
However, with the introduction of immersing lithography come technical challenges to be overcome if immersion lithography is to be a viable lithographic technique for defining sub-45 nm features.
One known immersion lithography apparatus comprises an illumination system to serve as a source of electromagnetic radiation. The illumination system is coupled to a support structure for holding a mask, the support structure being coupled to a first translation apparatus to position the illumination system accurately. A wafer table is disposed beneath the illumination system and is coupled to a second translation apparatus to position accurately the wafer table. A projection system is disposed adjacent the wafer table and projects light from the illumination system onto a wafer located on the wafer table. The projection system comprises an immersion head, which when in use, delivers and maintains an immersion liquid between the immersion head and the wafer.
In operation, the immersion lithography apparatus has a scan mode in which the wafer table is synchronously translated relative to the support structure, the immersion liquid having a leading edge corresponding to a direction of travel of the immersion liquid.
In order to increase wafer yields in connection with photolithographic processing of wafers, it is desirable to increase a velocity of translation of the wafer table relative to the support structure, i.e. to take less time to pattern the wafer. However, as scan rates increase to about 350 mms−1, it has been found that the leading edge of the immersion liquid rolls under the immersion liquid causing bubbles to form in the immersion liquid, which are then printed. Indeed, bubbles are known to be a significant hindrance to successful implementation of the immersion lithography technique. One known solution is to limit the scan rate, but this, of course, impacts negatively upon the achievable wafer yields.
Additionally, it is becoming desirable to use so-called “high n”, or high refractive index liquids as the immersion liquid. However, high n liquids need to be used in an oxygen-free environment, otherwise the refractive index of the high n begins to change rapidly. Further, due to the relative expense of high n liquids, the high n liquid is likely to be recycled in a closed system.